A structure provided with light-diffusing properties by adding fine particles can diffuse light emitted from a point light source such as an electric bulb and an LED or a line light source such as a fluorescent tube and allow the light to emit to a planar direction. Such structures have been extensively used as a backlight source for a liquid-crystal display device, and recently have been increasingly employed for other applications. Such applications include, for example, a light shielding plate as a window material utilizing its property that when a source light is OFF, it is transparent while when a source light is ON, it is opaque due to diffused light in a planar direction, and a light-emitting plate such as illumination, a decoration plate, a billboard and a sign. Examples of such structures which are widely used include a plate-like body made of an acrylic resin, a polycarbonate resin or the like to which fine particles are added, or which is surface-processed.
In addition to an acrylic resin, a polycarbonate resin or the like, a glass sheet is abrasion resistant and resistant to dust adhesion due to less static electricity, and is, therefore, sometimes used as a member constituting a light shielding plate and a light-emitting plate as described above. An example can be a transmissive screen employing a laminated glass employing an interlayer film containing spherical fine particles which satisfy particular requirements as a light-diffusion sheet, together with a Fresnel lens sheet/lenticular lens sheet made of an acrylic resin or the like (see Patent Reference No. 1). A material constituting the interlayer film can be a polyvinyl acetal represented by a polyvinyl butyral. Another example of a laminated glass utilizing diffusion light is a laminated glass which prevent a human or matter from being visually observed while allowing for transmission of visible light to some extent, and which comprises a plurality of glass sheets bonded via interlayer films containing titanium oxide fine particles (see Patent Reference No. 2).
However, an interlayer film made of a polyvinyl acetal has had problems: 1) tendency to discoloration due to heating, 2) tendency to generation of foreign materials (undissolved materials), and 3) tendency to deterioration in penetration resistance when it is used in a safety laminated glass. The problems 1) and 2) may lead to alteration in hue of a light shielding plate and a light-emitting plate, and failure to achieve even light-diffusing properties. The problem 3) may impair safety of a laminated glass. There have been various proposals for solving these problems. For example, Patent Reference Nos. 3 and 4 have described a method for preventing discoloration of a polyvinyl acetal by acetalization at a particular hydroxide ion concentration at an elevated temperature under a high pressure. Patent Reference No. 5 has described a method for preventing discoloration of a polyvinyl acetal obtained, by adding a reducing agent after acetalization and neutralization. However, for the methods described in Patent Reference Nos. 3 to 5, an interlayer film made of a polyvinyl acetal obtained tends to be discolored and to generate foreign materials. Furthermore, when the interlayer film is used in a safety laminated glass, penetration resistance tends to be deteriorated.
Furthermore, for providing a light shielding plate or light-emitting plate having even light-diffusing properties, it is necessary to highly disperse fine particles endowing with light-diffusing properties in an interlayer film made of a polyvinyl acetal. Patent Reference No. 6 has described a method for dispersing inorganic fine particles in an interlayer film made of a polyvinyl acetal by adding an acid-modified polyolefin. However, for a method described in Patent Reference No. 6, there is room for improving dispersibility of fine particles in an interlayer film made of a polyvinyl acetal. Thus, an interlayer film for a laminated glass made of a polyvinyl acetal whereby the above problems can be completely solved is strongly desired.